Optical fiber

ABSTRACT

An optical fiber includes a glass portion, a primary coating layer, and a secondary coating layer. In the optical fiber, a value of microbend loss characteristic factor FμBL_GΔβ is 6.1 ([GPa−1·μm−2.5/rad8]·10−12) or less when represented byFμBL_GΔβ=FμBL_G×FμBL_Δβ,where FμBL_G is geometry microbend loss characteristic and FμBL_Δβ is optical microbend loss characteristic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 17/297,716, filed May 27, 2021, and entitled “OPTICAL FIBER”.

TECHNICAL FIELD

The present invention relates to an optical fiber, specifically, to an optical fiber that can be used for an optical fiber cable.

BACKGROUND

In recent years, the traffic of communication infrastructures constructed by optical fiber cables and the like has been increasing due to the maturity of Fiber To The Home (FTTH) services, the spread of mobile terminals, the expansion of cloud service usage, the increase in video traffic, and the like. Therefore, it is demanded to construct communication infrastructures more economically and efficiently than before. Under such background, there is a demand to increase the number of mounting cores and mounting density of optical fibers mounted in optical fiber cables.

As a means for increasing the number of mounting cores and mounting density of the optical fibers, it is conceivable to reduce the diameter of the optical fibers. However, in this case, the optical fibers are easily affected by the lateral pressure, and the microbend loss, which is the optical loss caused by the shaft of the optical fibers being slightly bent, namely microbending, can be large. Patent Literature 1 below describes that the elastic modulus and the glass transition point of an optical fiber coating are adjusted to reduce the coating thickness of the optical fiber so that the microbend loss can be suppressed even when the diameter of the optical fiber is reduced.

-   [Patent Literature 1] JP 2012-508395 A

However, the aforementioned microbend loss tends to be affected by parameters related to the geometry of the optical fiber such as the coating thickness of the optical fiber, the outside diameter of the glass forming the core and the clad, the Young's modulus of the aforementioned glass, and the Young's modulus of the coating, and parameters related to the optical characteristics of the optical fiber such as the propagation constant of light propagating through the optical fiber. In Patent Literature 1 described above, the coating thickness is taken into consideration as the aforementioned parameter in terms of suppressing microbend loss, but the parameters other than the coating thickness are not taken into consideration. Therefore, there is a demand for an optical fiber capable of suppressing microbend loss that takes into consideration various parameters that affect microbend loss.

The present invention provides an optical fiber capable of suppressing microbend loss.

SUMMARY

One or more embodiments of the present invention provide an optical fiber including a glass portion including a core and a clad surrounding the core, a primary coating layer covering the clad, and a secondary coating layer covering the primary coating layer,

-   -   in which     -   a value of microbend loss characteristic factor F_(μBL_GΔβ)         ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) represented by         F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ)     -   by using     -   geometry microbend loss characteristic F_(μBL_G)         (GPa⁻¹·μm^(−10.5)·10⁻²⁷) of the optical fiber represented by

$F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}}$

when a spring coefficient of the primary coating layer is κs (MPa), a bending rigidity of the glass portion is H_(f) (MPa·μm⁴), a deformation resistance of the secondary coating layer is D₀ (MPa), a bending rigidity of the secondary coating layer is H₀ (MPa·μm⁴), a Young's modulus of the glass portion is E_(g) (GPa), a Young's modulus of the primary coating layer is E_(p) (MPa), a Young's modulus of the secondary coating layer is E_(s) (MPa), an outside diameter of the glass portion is d_(f) (μm), a radius of an outer peripheral surface of the primary coating layer is R_(p) (μm), a radius of an outer peripheral surface of the secondary coating layer is R_(s) (μm), a thickness of the primary coating layer is t_(p) (μm), and a thickness of the secondary coating layer is t_(s) (μm), and

optical microbend loss characteristic F_(μBL_Δβ) (1/(rad/μm)⁸) of the optical fiber represented by

$F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}$

when a difference between propagation constant of a waveguide mode propagating through the optical fiber and propagation constant of a radiation mode is propagation constant difference Δβ (rad/m),

is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.

The microbend loss of optical fiber is, as described in Non-Patent Literature 1 (J. Baldauf, et al., “Relationship of Mechanical Characteristics of Dual Coated Single Mode Optical Fibers and Microbending Loss,” IEICE Trans. Commun., vol. E76-B, No. 4, 1993.), Non-Patent Literature 2 (K. Petermann, et al., “Upper and Lower Limits for the Microbending Loss in Arbitrary Single-Mode Fibers,” J. Lightwave technology, vol. LT-4, no. 1, pp. 2-7, 1986.), Non-Patent Literature 3 (Okoshi et al. “Optical Fiber,” Ohmsha, pp. 235-239, 1989.), and Non-Patent Literature 4 (P. Sillard, et al., “Micro-Bend Losses of Trench-Assisted Single-Mode Fibers,” ECOC2010, We.8.F.3, 2010.), tends to be affected by both the geometry and the optical characteristics of the optical fiber.

Here, the geometry of the optical fiber is a parameter related to the structure of the optical fiber, and, in one or more embodiments of the present invention, the spring coefficient κs of a primary coating layer of the optical fiber, the bending rigidity H_(f) of a glass portion, the deformation resistance D₀ of a secondary coating layer, the bending rigidity H₀ of the secondary coating layer, the Young's modulus E_(g) of the glass portion, the Young's modulus E_(p) of the primary coating layer, the Young's modulus E_(s) of the secondary coating layer, the outside diameter d_(f) of the glass portion (diameter of the glass portion), the radius R_(p) of the primary coating layer, the radius R_(s) of the secondary coating layer, the thickness t_(p) of the primary coating layer, and the thickness t_(s) of the secondary coating layer.

By the way, according to Non-Patent Literature 2 to 4 described above, the microbend loss is a phenomenon caused by mode coupling in which a waveguide mode propagating through the optical fiber is coupled with a radiation mode. Such mode coupling is considered to occur due to the aforementioned microbending, and is said to be determined by a propagation constant difference (Δβ), which is the difference between the propagation constant of the waveguide mode of light propagating through the optical fiber and the propagation constant of the radiation mode. The above-mentioned optical characteristics of the optical fiber are parameters related to the characteristics of light propagating through the optical fiber, and in one or more embodiments of the present invention, mean the aforementioned propagation constant difference Δβ (rad/m).

The microbend loss of such an optical fiber may be represented by the value of sandpaper tension winding loss increase, which is the difference between the transmission loss measured in a state where the optical fiber is wound in one layer with a predetermined tension on a roughened bobbin body portion and the transmission loss measured in a state where the optical fiber is unwound from the bobbin with almost no tension applied. The smaller the value of such sandpaper tension winding loss increase becomes, the smaller is the microbend loss of the optical fiber.

By the way, as an optical fiber cable constituting communication infrastructures, a so-called tape slot type cable (RSCC: Ribbon Slotted Core Cable) including a plurality of tape core wires accommodated in each of a plurality of slots formed on a holding body for holding the tape core wires and a small-diameter high-density cable (UHDC: Ultra-High Density Cable) including tape core wires densely arranged inside the cable without using the aforementioned holding body are known. Of these, since the tape slot type cable has a structure in which a plurality of tape core wires are accommodated in the slots as described above, a lateral pressure is applied to the optical fibers constituting the tape core wires, which may cause microbend loss. Therefore, in the tape slot type cable, in consideration of such microbend loss, an optical fiber in which the value of sandpaper tension winding loss increase is suppressed to 0.6 dB/km or less may be used.

The present inventor has diligently studied the relationship between the sandpaper tension winding loss increase and the aforementioned various parameters regarding the optical fiber used for the optical fiber cable. As a result, it was found that the value of microbend loss characteristic factor F_(μBL_GΔβ) represented by the aforementioned formula has a high correlation with the value of sandpaper tension winding loss increase. That is, the present inventor has found that the value of the microbend loss characteristic factor has a substantially positive slope proportional relationship with the value of sandpaper tension winding loss increase.

Furthermore, the present inventor has conducted further research and found that when the value of the aforementioned microbend loss characteristic factor is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²), the value of the sandpaper tension winding loss increase is a value slightly smaller than 0.6 dB/km. As described above, the value of the microbend loss characteristic factor has a substantially positive slope proportional relationship with the value of sandpaper tension winding loss increase. Therefore, by setting the value of the microbend loss characteristic factor of the optical fiber to 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, the microbend loss can be suppressed to the extent that the optical fiber can be applied to the tape slot type cable.

As described above, with the optical fiber of one or more embodiments of the present invention, the microbend loss can be suppressed.

Furthermore, in the aforementioned optical fiber, the value of microbend loss characteristic factor may be 5.8 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.

Furthermore, the value of the microbend loss characteristic factor may be 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.

Furthermore, in the aforementioned optical fiber, the value of microbend loss characteristic factor may be 3.75 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.

Among the optical fiber cables constituting the communication infrastructures, in the small-diameter high-density cable, the tape core wires are densely arranged as described above. Therefore, similar to the tape slot type cable, the optical fiber constituting the tape core wire is subjected to a lateral pressure, and the microbend loss can occur. Furthermore, since the small-diameter high-density cable is slotless as described above and all the tape core wires are densely arranged inside the cable, the optical fiber tends to be subjected to a large lateral pressure as compared with the tape slot type cable in which the tape core wires are separately arranged in a plurality of grooves. Therefore, in the small-diameter high-density cable, it is recommended to use an optical fiber having a smaller microbend loss than the optical fiber used for the tape slot type cable. In view of the above, in the small-diameter high-density cable, an optical fiber in which the value of sandpaper tension winding loss increase is suppressed to 0.34 dB/km or less may be used.

The present inventor has found that the value of the microbend loss characteristic factor substantially corresponding to the value (0.34 dB/km) of the sandpaper tension winding loss increase is 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²). Therefore, by setting the value of the microbend loss characteristic factor of the optical fiber to 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, the microbend loss can be suppressed to the extent that the optical fiber can also be applied to the small-diameter high-density cable.

Furthermore, in the aforementioned optical fiber, the coating thickness of a sum of the thickness of the primary coating layer and the thickness of the secondary coating layer may be 42.0 μm or less.

The larger the aforementioned coating thickness becomes, the larger the outside diameter of the optical fiber tends to be, and the smaller the coating thickness becomes, the smaller the outside diameter of the optical fiber tends to be. The optical fiber used for the optical fiber cable constituting the communication infrastructures generally has a coating thickness of approximately 60 μm. Therefore, when the coating thickness is 42.0 μm or less, it is possible to realize an optical fiber having a smaller diameter than a general optical fiber constituting the communication infrastructures. By the way, the value of the microbend loss characteristic factor is determined by various parameters as described above, and the parameters include the thickness of the primary coating layer and the thickness of the secondary coating layer. Therefore, according to one or more embodiments of the present invention, even when the thickness of the primary coating layer or the thickness of the secondary coating layer is reduced, the value of the microbend loss characteristic factor can be 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less or can be 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less by adjusting other parameters. Therefore, even when the coating thickness of the optical fiber of the present invention is 42.0 μm or less, the microbend loss can be suppressed to the extent that the optical fiber can be used for the tape slot type cable or the small-diameter high-density cable.

Furthermore, the outside diameter of the glass portion may be 65 μm or more and 100 μm or less.

The larger the outside diameter of the aforementioned glass portion becomes, the larger the outside diameter of the optical fiber tends to be, and the smaller the outside diameter of the glass portion becomes, the smaller the outside diameter of the optical fiber tends to be. The optical fiber used for the optical fiber cable constituting the communication infrastructures is generally formed such that the outside diameter of the glass portion is 125 μm. Therefore, when the outside diameter of the glass portion is 100 μm or less, it is possible to realize an optical fiber having a smaller diameter than a general optical fiber constituting the communication infrastructures. By the way, the value of the microbend loss characteristic factor is determined by various parameters as described above, and the parameters include the coating thickness and the outside diameter of the glass portion. Therefore, according to one or more embodiments of the present invention, even when the coating thickness is reduced and the outside diameter of the glass portion is reduced, the value of the microbend loss characteristic factor can be 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less or can be 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less by adjusting other parameters. Therefore, even when the coating thickness of the optical fiber of the present invention is 42.0 μm or less and the outside diameter of the glass portion is 100 μm or less, the microbend loss can be suppressed to the extent that the optical fiber can be used for the tape slot type cable or the small-diameter high-density cable.

Note that when the outside diameter of a glass portion having brittleness is as thin as approximately 65 μm, the mechanical bending resistance of the optical fiber can be increased by the amount that the brittle glass is thinned.

Furthermore, in the aforementioned optical fiber, the Young's modulus of the primary coating layer may be 0.08 MPa or more and 0.70 MPa or less.

Furthermore, in the aforementioned optical fiber, the Young's modulus of the secondary coating layer may be 800 MPa or more and 1400 MPa or less.

Furthermore, in the aforementioned optical fiber, a propagation constant difference between LP01 mode and LP11 mode in light having a wavelength of 1550 nm may be 10446 rad/m or more and 15702 rad/m or less.

Furthermore, in the aforementioned optical fiber, a MAC value (a.u.) may be 5.87 or more and 7.57 or less.

Furthermore, in the aforementioned optical fiber, a mode field diameter of light having a wavelength of 1310 nm may be 7.6 μm or more and 9.12 μm or less.

Furthermore, in the aforementioned optical fiber, a mode field diameter of light having a wavelength of 1310 nm may be 7.6 μm or more and less than 8.6 μm.

Furthermore, in the aforementioned optical fiber, a zero dispersion wavelength may be 1300 nm or more and 1324 nm or less.

Furthermore, in the aforementioned optical fiber, a zero dispersion slope may be 0.073 ps/km/nm² or more and 0.092 ps/km/nm² or less.

Furthermore, the mode field diameter at a wavelength of 1310 nm may be 7.6 μm or more and 9.12 μm or less, the cable cutoff wavelength may be 1260 nm or less, the zero dispersion wavelength may be 1300 nm or more and 1324 nm or less, and the zero dispersion slope may be 0.073 ps/km/nm² or more and 0.092 ps/km/nm² or less.

In this case, the macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm may be 1.5 dB/turn or less.

Furthermore, in the aforementioned optical fiber, the value of microbend loss characteristic factor is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less (excluding 5.83 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) and 5.99 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²)).

Furthermore, in the aforementioned optical fiber, the value of microbend loss characteristic factor is less than 5.83 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²).

As described above, according to one or more embodiments of the present invention, there is provided an optical fiber capable of suppressing the microbend loss.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing a structure of a cross section perpendicular to a longitudinal direction of an optical fiber cable according to a first embodiment of the present invention.

FIG. 2 is a perspective view schematically showing an example of an optical fiber tape core wire included in the optical fiber cable shown in FIG. 1 .

FIG. 3 is a diagram schematically showing a structure of a cross section perpendicular to the longitudinal direction of the optical fiber included in the optical fiber tape core wire shown in FIG. 2 .

FIG. 4 is a diagram showing a structure of a cross section perpendicular to a longitudinal direction of an optical fiber cable according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between a value of microbend loss characteristic factor and sandpaper tension winding loss increase in the optical fibers shown in the samples 1 to 22.

FIG. 6 is a diagram showing the relationship between the value of the microbend loss characteristic factor and sandpaper tension winding loss increase in the optical fibers shown in the samples 1 to 22 and 49 to 57.

DETAILED DESCRIPTION

Aspects for carrying out the optical fiber according to the present invention will be illustrated below together with the accompanying drawings. The embodiments illustrated below are for facilitating the understanding of the present invention, and are not for limiting the interpretation of the present invention. The present invention can be changed or modified from the embodiments below without departing from the spirit. Furthermore, in the present specification, the dimensions of each member may be exaggerated for ease of understanding.

First Embodiment

FIG. 1 is a diagram schematically showing a structure of a cross section perpendicular to a longitudinal direction of an optical fiber cable 1 according to the first embodiment. As shown in FIG. 1 , the optical fiber cable 1 is a so-called tape slot type cable (RSCC: Ribbon Slotted Core Cable). The optical fiber cable 1 includes a sheath 3, a plurality of tape core wires 4, a holding body 5, and a tensile strength body 6 as main configurations.

The sheath 3 is a tubular member and may be formed of a thermoplastic resin such as polyethylene. The aforementioned holding body 5 is accommodated in the internal space of the sheath 3. In this way, the sheath 3 accommodates the holding body 5 inside and protects the holding body 5.

The holding body 5 is a member that holds the plurality of tape core wires 4. A plurality of slots 5S are formed on the holding body 5, and the plurality of tape core wires 4 are accommodated in the slots 5S. Note that by increasing the number of tape core wires 4 accommodated in the slots 5S, the number of cores of optical fiber included in the optical fiber cable 1 can be increased.

In the present embodiment, the tensile strength body 6 is embedded in the substantially center of the holding body 5 in the cross-sectional view of FIG. 1 . Such tensile strength body 6 can suppress the tape core wires 4 from being stretched more than necessary when tension is applied in the longitudinal direction of the tape core wires 4.

FIG. 2 is a perspective view schematically showing an example of the tape core wire 4. As shown in FIG. 2 , the tape core wire 4 of the present embodiment is a so-called intermittent adhesive type tape core wire. The tape core wire 4 has a configuration in which a plurality of optical fibers are arranged along a direction perpendicular to the longitudinal direction, and the arranged optical fibers 10 are adhered to each other. The tape core wire 4 includes adhesive portions 4A and single core portions 4B. The adhesive portion 4A is a portion where adjacent optical fibers 10 are adhered to each other, and is provided intermittently at a constant pitch along the longitudinal direction. The single core portion 4B is a portion located between the adhesive portions 4A, and is a portion where the optical fibers 10 are not adhered to each other. With such a configuration, the tape core wire 4 can be easily deformed, for example, twisted or bundled in a substantially cylindrical shape. FIG. 1 schematically shows a state in which each tape core wire 4 is bundled in a substantially cylindrical shape.

Note that FIG. 2 shows an example in which the tape core wire 4 includes four optical fibers 10, but this is an example. That is, the number of optical fibers 10 constituting the tape core wire 4 is not particularly limited, and may be less than four or may be more than four. For example, the tape core wire 4 may include 12-core optical fibers 10. Furthermore, the tape core wire 4 is not limited to the intermittent adhesive type.

FIG. 3 is a diagram showing a structure of a cross section perpendicular to the longitudinal direction of the optical fiber 10 constituting the tape core wire 4. The optical fiber 10 of the present embodiment is a single-mode optical fiber. As shown in FIG. 3 , the optical fiber 10 includes a core 11, a clad 12 that surrounds the core 11 without gaps, a primary coating layer 14 that covers the clad 12, and a secondary coating layer 15 that covers the primary coating layer 14 as the main configurations. In the optical fiber 10, the clad 12 has a lower refractive index than the core 11.

The core 11 may be formed of pure quartz to which no dopant has been added, or may be formed of quartz to which germanium (Ge) or the like that increases the refractive index has been added as a dopant.

The clad 12 has a lower refractive index than the core 11 as described above. For example, when the core 11 is formed of pure quartz, the clad 12 may be formed of quartz to which fluorine (F), boron (B), or the like that lowers the refractive index has been added as a dopant, and when the core 11 is formed of quartz to which germanium (Ge) or the like that increases the refractive index has been added as a dopant, the clad 12 may be formed of pure quartz to which no dopant has been added. Furthermore, the clad 12 may be formed of quartz to which chlorine (C12) has been added. Furthermore, the clad 12 may be a single layer, may include a plurality of layers having different refractive indexes, or may be a hole-assisted type.

As described above, the core 11 and the clad 12 are both formed of quartz (glass). Therefore, the core 11 and the clad 12 are collectively referred to as a glass portion 13. That is, the glass portion 13 includes the core 11 and the clad 12, and the glass portion 13 is covered with the primary coating layer 14. Note that the glass portion 13 is sometimes also referred to as an optical fiber bare wire portion. The outside diameter (diameter) d_(f) of such glass portion 13 is generally 125 μm. However, in the present embodiment, the outside diameter d_(f) of the glass portion 13 can be a smaller outside diameter. For example, it can be 65 μm or more and 100 μm or less, 65 μm or more and 90 μm or less, 65 μm or more and 80 μm or less, 65 μm or more and 75 μm or less, or 65 μm or more and 70 μm or less. The reason why the outside diameter d_(f) of the glass portion 13 can be small in this way will be described later.

Note that when the outside diameter d_(f) of a glass portion having brittleness is as thin as approximately 65 μm, the mechanical bending resistance of the optical fiber can be increased by the amount that the brittle glass is thinned.

The primary coating layer 14 is formed of, for example, an ultraviolet curable resin or a thermosetting resin, and is formed on an outer side of the glass portion 13 with a thickness t_(p) (μm). In the present embodiment, the Young's modulus E_(g) of the primary coating layer 14 is lower than the Young's modulus E_(s) of the secondary coating layer 15. By setting the primary coating layer 14 in direct contact with the glass portion to have a low Young's modulus in this way, the primary coating layer 14 acts as a cushioning material, and the external force acting on the glass portion 13 can be reduced. Note that assuming that the radius of the outer peripheral surface of the primary coating layer 14 is R_(p) (μm), the outside diameter of the primary coating layer 14 is represented by 2R_(p), and assuming that the radius (d_(f)×½) of the glass portion is R_(g) (μm), the aforementioned thickness t_(p) of the primary coating layer 14 is represented by the formula described below. t _(p) =R _(p) −R _(g)

In the present embodiment, the secondary coating layer 15 is a layer forming the outermost layer of the optical fiber 10, and is formed of, for example, an ultraviolet curable resin or a thermosetting resin different from the type of resin forming the primary coating layer 14, and is formed on an outer side of the primary coating layer 14 with a thickness t_(s) (μm). For example, when the primary coating layer 14 is formed of an ultraviolet curable resin, the secondary coating layer 15 may be formed of an ultraviolet curable resin different from the ultraviolet curable resin forming the primary coating layer 14, and when the primary coating layer 14 is formed of a thermosetting resin, the secondary coating layer 15 may be formed of a thermosetting resin different from that of the primary coating layer 14. In the present embodiment, the Young's modulus E_(s) of the secondary coating layer 15 is higher than the Young's modulus E_(g) of the primary coating layer 14. As described above, by setting the secondary coating layer 15 forming the outermost layer of the optical fiber 10 to have a high Young's modulus, the glass portion 13 can be appropriately protected from the external force. Note that assuming that the radius of the outer peripheral surface of the secondary coating layer 15 is R_(s), the outside diameter of the secondary coating layer 15, i.e., the outside diameter of the optical fiber 10 is represented by 2R_(s), and the aforementioned thickness t_(s) of the secondary coating layer 15 is represented by the formula described below. t _(s) =R _(s) −R _(p)

By the way, the outside diameter of the optical fiber used for the optical fiber cable is generally approximately 240 μm to 250 μm. Therefore, the outside diameter of the secondary coating layer 15 may be approximately 240 μm. However, in the present embodiment, the outside diameter of the secondary coating layer 15 can be smaller than 240 μm. For example, it can be approximately 190 μm, approximately 150 μm to approximately 160 μm, or approximately 125 μm. The reason why the outside diameter of the secondary coating layer 15, i.e., the outside diameter of the optical fiber 10 can be small in this way will be described later.

Furthermore, assuming that the sum of the thickness t_(p) of the primary coating layer 14 and the thickness t_(s) of the secondary coating layer 15 is the coating thickness t, the coating thickness of the optical fiber used for the optical fiber cable is generally approximately 60 μm. Therefore, the coating thickness t of the optical fiber 10 may be approximately 60 μm. However, in the present embodiment, the coating thickness t of the optical fiber 10 can be smaller than 60 μm. For example, it can be 42.5 μm or less, 38.0 μm or less, 36.5 μm or less, 34.5 μm or less, or 34.0 μm or less. The reason why the coating thickness of the optical fiber 10 can be small in this way will be described later.

As described above, in the optical fiber cable 1 of the present embodiment, the tape core wires 4 including a plurality of such optical fibers 10 are densely accommodated in the slots 5S of the holding body 5. As a result, the optical fiber cable 1 can accommodate a large number of cores of optical fiber. For example, the optical fiber cable 1 accommodates 1000-core or more optical fibers. Furthermore, as described above, in the optical fiber 10 of the present embodiment, the glass portion 13 can be formed to have an outside diameter smaller than that of the glass portion of a general optical fiber, and the coating thickness can be formed to be smaller than the coating thickness of a general optical fiber. Therefore, the outside diameter of the optical fiber 10 can be smaller than the outside diameter of a general optical fiber, and the diameter of the optical fiber 10 can be reduced. By reducing the diameter of the optical fiber 10 in this way, the size of the tape core wire 4 can be smaller than the size of a general tape core wire. Therefore, the tape core wires 4 having such a small size are accommodated in the slots 5S, and the number of cores of optical fiber accommodated in the optical fiber cable 1 can be further increased. Alternatively, the tape core wires 4 having such a small size are accommodated in the slots 5S, and the size of the optical fiber cable 1 can be reduced.

On the other hand, as the accommodation density of the tape core wires in the slots increases, the lateral pressure acting on the optical fiber tends to increase. When the optical fiber receives the lateral pressure in this way, the shaft of the optical fiber is slightly bent, and the microbend loss can occur. Furthermore, when the outside diameter of the glass portion of the optical fiber or the coating thickness of the optical fiber is reduced, the glass portion is susceptible to the lateral pressure, and eventually the microbend loss can occur.

However, the optical fiber 10 of the present embodiment is formed so that the value of the microbend loss characteristic factor F_(μBL_GΔβ) described later is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less. Therefore, even when the outside diameter of the glass portion and the coating thickness are reduced and the number of cores of the optical fiber 10 accommodated in the slots 5S is increased, the microbend loss can be suppressed. The reason for this will be described in detail below.

The microbend loss of optical fiber is, as described in Non-Patent Literature 1 (J. Baldauf, et al., “Relationship of Mechanical Characteristics of Dual Coated Single Mode Optical Fibers and Microbending Loss,” IEICE Trans. Commun., vol. E76-B, No. 4, 1993.), Non-Patent Literature 2 (K. Petermann, et al., “Upper and Lower Limits for the Microbending Loss in Arbitrary Single-Mode Fibers,” J. Lightwave technology, vol. LT-4, no. 1, pp. 2-7, 1986.), Non-Patent Literature 3 (Okoshi et al. “Optical Fiber,” Ohmsha, pp. 235-239, 1989.), and Non-Patent Literature 4 (P. Sillard, et al., “Micro-Bend Losses of Trench-Assisted Single-Mode Fibers,” ECOC2010, We.8.F.3, 2010.), tends to be affected by both the geometry and the optical characteristics of the optical fiber.

Here, the geometry of the optical fiber is a parameter related to the structure of the optical fiber, and is, in the present embodiment, the spring coefficient κs of the primary coating layer of the optical fiber, the bending rigidity H_(f) of the glass portion, the deformation resistance D₀ of the secondary coating layer, the bending rigidity H₀ of the secondary coating layer, the Young's modulus E_(g) of the glass portion, the Young's modulus E_(p) of the primary coating layer, the Young's modulus E_(s) of the secondary coating layer, the outside diameter d_(f) of the glass portion (diameter of the glass portion), the radius R_(p) of the primary coating layer, the radius R_(s) of the secondary coating layer, the thickness t_(p) of the primary coating layer, and the thickness t_(s) of the secondary coating layer.

By the way, according to Non-Patent Literature 2 to 4 described above, the microbend loss is a phenomenon caused by mode coupling in which a waveguide mode propagating through the optical fiber is coupled with a radiation mode. This waveguide mode is, for example, LP01 mode. Such mode coupling is said to occur due to the slight bending of the shaft of the optical fiber, namely microbending, and is considered to be determined by the propagation constant difference (Δβ), which is the difference between the propagation constant in the waveguide mode and the propagation constant in the radiation mode. The above-mentioned optical characteristics of the optical fiber are parameters related to the characteristics of light propagating through the optical fiber, and in the present invention, mean the aforementioned propagation constant difference Δβ (rad/m).

The microbend loss of such an optical fiber may be represented by the value of sandpaper tension winding loss increase, which is the difference between the transmission loss measured in a state where the optical fiber is wound in one layer with a predetermined tension on a roughened bobbin body portion and the transmission loss measured in a state where the optical fiber is unwound from the bobbin with almost no tension applied. The smaller the value of such sandpaper tension winding loss increase becomes, the smaller is the microbend loss of the optical fiber.

By the way, the tape slot type cable (RSCC) such as the optical fiber cable 1 of the present embodiment can cause the microbend loss as described above. Therefore, the tape slot type cable has the required characteristics that the value of sandpaper tension winding loss increase is 0.6 dB/km or less in consideration of such microbend loss.

The relationship between the sandpaper tension winding loss increase and the aforementioned various parameters regarding the optical fiber used for the optical fiber cable was studied. As a result, it was found that a value of microbend loss characteristic factor F_(μBL_GΔβ) represented by the formula (3) below F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ)  (3)

-   -   by using     -   geometry microbend loss characteristic F_(μBL_G) determined by         the formula (1) below

$\begin{matrix} {F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{1.125 - {0.25\mu}} \times H_{0}^{{0.625\mu} - 0.125}}} & (1) \end{matrix}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}}$

-   -   related to a spring coefficient κs of the primary coating layer,         a bending rigidity H_(f) of the glass portion, a deformation         resistance D₀ of the secondary coating layer, a bending rigidity         H₀ of the secondary coating layer, a Young's modulus E_(g) of         the glass portion, a Young's modulus E_(p) of the primary         coating layer, a Young's modulus E_(s) of the secondary coating         layer, an outside diameter d_(f) of the glass portion, a radius         R_(p) of the outer peripheral surface of the primary coating         layer, a radius R_(s) of the outer peripheral surface of the         secondary coating layer, a thickness t_(p) of the primary         coating layer, and a thickness t_(s) of the secondary coating         layer, which are parameters related to geometry,     -   and     -   optical microbend loss characteristic F_(μBL_Δβ) determined by         the formula (2) below

$\begin{matrix} {F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}} & (2) \end{matrix}$

-   -   related to a propagation constant difference Δβ, which is a         parameter related to optical characteristics,     -   has a high correlation with the value of sandpaper tension         winding loss increase. That is, the present inventor has found         that the value of the microbend loss characteristic factor has a         substantially positive slope proportional relationship with the         value of sandpaper tension winding loss increase.

Note that according to Non-Patent Literature 5 (K. Kobayashi, et al., “Study of Microbending loss in thin coated fibers and fiber ribbons,” IWCS, pp. 386-392, 1993.), the typical value of the constant μ in the aforementioned formula (1) is “3”. Therefore, the aforementioned formula (1) becomes the formula (4) described below.

$\begin{matrix} {F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}} & (4) \end{matrix}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}}$

Furthermore, according to Non-Patent Literature 2 described above and Non-Patent Literature 6 (C. D. Hussey, et al., “Characterization and design of single-mode optical fibres,” Optical and Quantum Electronics, vol. 14, no. 4, pp. 347-358, 1982.), the typical value of the constant p in the aforementioned formula (2) is “4”. Therefore, the aforementioned formula (2) becomes the formula (5) described below.

$\begin{matrix} {F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}} & (5) \end{matrix}$

Furthermore, the present inventor has conducted further research and found that when the value of the aforementioned microbend loss characteristic factor is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²), the value of the sandpaper tension winding loss increase is a value slightly smaller than 0.6 dB/km. As described above, the value of the microbend loss characteristic factor has a substantially positive slope proportional relationship with the value of sandpaper tension winding loss increase. Therefore, by setting the value of the microbend loss characteristic factor of the optical fiber to 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, the microbend loss can be suppressed to the extent that the required characteristics of the tape slot type cable are satisfied.

As described above, the optical fiber 10 of the present embodiment is formed so that the value of the microbend loss characteristic factor F_(μBL_GΔβ) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less. Therefore, in the optical fiber 10 of the present embodiment, the microbend loss can be suppressed to the extent that the required characteristics of the tape slot type cable are satisfied. Therefore, the optical fiber cable 1 using the optical fiber 10 can exhibit favorable optical characteristics.

Furthermore, as described above, in the optical fiber 10 of the present embodiment, even when the outside diameter d_(f) of the glass portion 13 is made smaller than 125 μm or the coating thickness t is made smaller than 60 μm, because parameters other than the outside diameter d_(f) of the glass portion and the coating thickness t are adjusted so that the value of the microbend loss characteristic factor F_(μBL_GΔβ) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, the microbend loss can be suppressed to the extent that the required characteristics of the tape slot type cable are satisfied. Here, as shown in FIG. 3 , the outside diameter 2R_(s) of the optical fiber 10 is represented by 2R _(s) =d _(f)+2t using the outside diameter d_(f) of the glass portion and the coating thickness t. Therefore, as described above, the diameter of the optical fiber can be reduced by reducing the coating thickness t and the outside diameter d_(f) of the glass portion. Therefore, by using the optical fiber 10 whose diameter is reduced and microbend loss is suppressed in this way, a tape slot type cable having excellent optical characteristics that realizes an increase in the number of cores and a small size can be configured.

Second Embodiment

Next, the second embodiment will be described with reference to FIG. 4 . FIG. 4 is a diagram schematically showing a structure of a cross section perpendicular to a longitudinal direction of an optical fiber cable 2 according to the present embodiment. Note that the same or equivalent components as those of the first embodiment are designated by the same reference numerals and duplicated description will be omitted unless otherwise specified.

As shown in FIG. 4 , the optical fiber cable 2 of the present embodiment has the same configuration as the optical fiber cable 1 of the first embodiment in that tape core wires 4 having substantially the same configuration as that of the first embodiment are accommodated inside. However, the optical fiber cable 2 is mainly different from the optical fiber cable 1 on the points described below.

The optical fiber cable 1 is a tape slot type cable (RSCC) as described above. On the other hand, as shown in FIG. 4 , the optical fiber cable 2 of the present embodiment does not have the holding body 5. That is, the optical fiber cable 2 is a so-called small-diameter high-density cable (UHDC: Ultra-High Density Cable) in which the tape core wires are not accommodated in the slots of the holding body but are directly accommodated in the sheath. That is, an accommodation space 3S is formed inside the sheath 3 of the optical fiber cable 2, and a plurality of tape core wires 4 are arranged in the accommodation space 3S. Note that the tensile strength bodies 6 may be embedded in the sheath 3 of the optical fiber cable 2 at positions facing each other across the center of the optical fiber cable 2.

Furthermore, as described above, the tape core wire 4 of the present embodiment has substantially the same configuration as the tape core wire 4 of the first embodiment. However, the value of the microbend loss characteristic factor F_(μBL_GΔβ) of the optical fiber 10 included in the tape core wire 4 of the present embodiment is 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less for the reason described later.

Since the small-diameter high-density cable such as the optical fiber cable 2 does not have the holding body 5 as described above and is slotless, the tape core wires 4 can be densely arranged in the accommodation space 3S of the sheath 3. Therefore, a large number of tape core wires can be accommodated as compared with the tape slot type cable such as the optical fiber cable 1.

On the other hand, in the small-diameter high-density cable, since many tape core wires are densely arranged in one place as described above, a large lateral pressure tends to be applied to the optical fiber as compared with the tape slot type cable. Therefore, in the small-diameter high-density cable, it is recommended to use an optical fiber having a smaller microbend loss than the optical fiber used for the tape slot type cable. In view of the above, the small-diameter high-density cable has the required characteristics that the value of the aforementioned sandpaper tension winding loss increase is 0.34 dB/km or less.

The present inventor has calculated the value of the microbend loss characteristic factor F_(μBL_GΔβ) corresponding to the value (0.34 dB/km) of the sandpaper tension winding loss increase on the basis of the aforementioned formulae (3) to (5) and found that the value is 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²). That is, it has been found that by setting the value of the microbend loss characteristic factor F_(μBL_GΔβ) to 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, the microbend loss can be suppressed to the extent that the required characteristics of the small-diameter high-density cable are satisfied.

As described above, the optical fiber 10 of the present embodiment is configured so that the aforementioned various parameters are adjusted so that the value of the microbend loss characteristic factor F_(μBL_GΔβ) is 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less. Therefore, the microbend loss can be suppressed to the extent that the required characteristics of the small-diameter high-density cable are satisfied. Therefore, the optical fiber cable 2 using the optical fiber 10 can exhibit favorable optical characteristics.

Furthermore, as described above, in the optical fiber 10 of the present embodiment, even when the optical fiber 10 has a reduced diameter by making the outside diameter d_(f) of the glass portion 13 smaller than 125 μm and the coating thickness t smaller than 60 μm, the microbend loss can be suppressed to the extent that the required characteristics of the small-diameter high-density cable are satisfied. Therefore, by using the optical fiber 10 whose diameter is reduced in this way, a small-diameter high-density cable having excellent optical characteristics that realizes an increase in the number of cores and a small size can be configured.

Next, the reason why the outside diameter d_(f) of the glass portion 13 can be small, the reason why the coating thickness of the optical fiber 10 can be small, the reason why the outside diameter of the optical fiber 10 can be small, and the like will be described.

The present inventor performed the following Examples 1 to 48 in order to verify the relationship between the value of the microbend loss characteristic factor F_(μBL_GΔβ) and the value of the sandpaper tension winding loss increase α_(μBL). Note that aspects for carrying out the present invention are not limited to Examples 1 to 48.

Examples 1 to 22

The present inventor prepared Samples 1 to 22 of optical fiber in which the aforementioned various parameters were changed, measured the value of sandpaper tension winding loss increase for each of Samples 1 to 22, and calculated the value of the microbend loss characteristic factor F_(μBL_GΔβ) on the basis of the aforementioned formulae (3) to (5). The optical fiber of Sample 1 is the optical fiber of Example 1, and the optical fiber of Sample 2 is the optical fiber of Example 2. Thus, the sample numbers of the optical fibers correspond to the numbers of the Examples. Note that the optical fiber of Sample 8 is an optical fiber generally used for an optical fiber cable constituting the communication infrastructures, and has an outside diameter of the glass portion of 125 μm and a coating thickness of 57.5 μm. The optical fiber of Sample 8 may be referred to as the “general optical fiber”.

The test for sandpaper tension winding loss increase was performed in the manner described below. That is, first, sandpaper (SiC having an average particle diameter of 50 μm (for example, model number #360)) was wound around the bobbin body portion having a body diameter of 380 mm, and one layer of optical fiber wire was wound therearound at 100 gf, and light having a wavelength of 1550 nm was caused to propagate. The transmission loss at this time was measured. Thereafter, the optical fiber wire was unwound from the bobbin, light having a wavelength of 1550 nm was caused to propagate with almost no tension applied, and the transmission loss was measured. Then, the difference between these transmission losses was obtained, and the value of this difference was defined as sandpaper tension winding loss increase α_(μBL).

Tables 1 to 5 below show the parameter specifications for each of Samples 1 to 22, the values of the microbend loss characteristic factor F_(μBL_GΔβ) for each of Samples 1 to 22, and the values of the sandpaper tension winding loss increase α_(μBL) for each of Samples 1 to 22.

Note that in Tables 1 to 5 below and Tables 7 to 10 described below, mode field diameter (MFD), cutoff wavelength, macrobend loss, and the like are as described below. The mode field diameter is the mode field diameter of light in the LP01 mode when light having a wavelength of 1310 nm is caused to propagate through the optical fiber.

Note that the mode field diameter is expressed by Petermann II definition formula (formula (6) below) in ITU-T Recommendation G.650.1. Here, E(r) represents the electric field strength at the point where the distance from the central axis of the optical fiber is r.

$\begin{matrix} {{MFD} = {{2w} = {2\sqrt{\frac{2{\int\limits_{0}^{\infty}{{E^{2}(r)}{rdr}}}}{\int\limits_{0}^{\infty}{\left\lbrack {{{dE}(r)}/{dr}} \right\rbrack^{2}{rdr}}}}}}} & (6) \end{matrix}$

Furthermore, the aforementioned cutoff wavelength indicates the minimum wavelength at which the high-order mode is sufficiently attenuated. This high-order mode refers to, for example, LP11 mode. Specifically, it is the minimum wavelength at which the loss of the high-order mode is 19.3 dB. The cutoff wavelength includes a fiber cutoff wavelength and a cable cutoff wavelength, and can be measured by, for example, the measurement method described in ITU-T Recommendation G.650. The cutoff wavelengths described in Tables 1 to 5 are cable cutoff wavelengths. Furthermore, the MAC value is the ratio of the mode field diameter of light at a wavelength of 1310 nm to the cable cutoff wavelength, and is defined as 2w/λ_(cc) when the mode field diameter is defined as 2w and the cable cutoff wavelength is defined as λ_(cc). Furthermore, the macrobend losses are the bend loss caused by light having a wavelength of 1625 nm propagating through a bent portion formed when the optical fiber is bent with a radius of 10 mm. The unit “/turn” of macrobend loss means “per turn of optical fiber”. Furthermore, the propagation constant difference is the difference between the propagation constant of light having a wavelength of 1550 nm in the waveguide mode and the propagation constant of light having a wavelength of 1550 in the radiation mode, and, in this experiment, is the difference between the propagation constant of light having a wavelength of 1550 nm in the LP01 mode and the propagation constant in the LP11 mode. The propagation constant was calculated using the two-dimensional finite element method described in Non-Patent Literature 7 (K. Saitoh and M. Koshiba, “Full-Vectorial Imaginary-Distance Beam Propagation Method Based on a Finite Element Scheme: Application to Photonic Crystal Fibers,” IEEE J. Quant. Elect. vol. 38, pp. 927-933, 2002.) on the basis of the refractive index profile of a prototyped optical fiber. Furthermore, the zero dispersion wavelength refers to a wavelength at which the value of the wavelength dispersion becomes zero. Here, the wavelength dispersion is the sum of material dispersion and waveguide dispersion. Furthermore, the zero dispersion slope refers to the rate of change of wavelength dispersion with respect to the wavelength at the zero dispersion wavelength.

TABLE 1 Example 1 2 3 4 5 Outside diameter of 80 80 80 125 80 glass portion(μm) Outside diameter of 125 115 115 159 115 primary coating layer(μm) Outside diameter of 156 152 164 193 153 secondary coating layer(μm) Young's modulus of 74 74 74 74 74 glass portion(GPa) Young's modulus of 0.2 0.2 0.2 0.5 0.2 primary coating layer(MPa) Young's modulus of 1150 1400 1400 1150 1400 secondary coating layer(MPa) Thickness of primary 22.5 17.5 17.5 17 17.5 coating layer(μm) Thickness of secondary 15.5 18.5 24.5 17 19 coating layer(μm) Coating thickness(μm) 38 36 42 34 36.5 Bending rigidity of 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 8.87 × 10¹¹ 1.49 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 1.97 × 10¹⁰ 2.47 × 10¹⁰ 3.77 × 10¹⁰ 4.22 × 10¹⁰ 2.56 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 κ_(s)(MPa) 0.71 0.91 0.91 3.68 0.91 Deformation resistance of 9.22 20.39 37.54 6.79 21.65 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 3.66 3.90 2.38 1.92 3.72 10⁻²⁷) Mode field diameter(μm) 8.83 8.66 8.66 8.6 8.6 Cable cutoff wavelength(μm) 1.230 1.230 1.230 1.230 1.200 MAC value(a.u.) 7.18 7.04 7.04 6.99 7.17 Macrobend loss(dB/turn) 0.100 0.050 0.050 0.050 0.200 Propagation constant 12498 13182 13182 13066 11613 difference (rad/m) Zero dispersion 1.311 1.316 1.316 1.317 1.310 wavelength(μm) Zero dispersion 0.088 0.086 0.086 0.086 0.087 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 1.68 × 10¹⁵ 1.10 × 10¹⁵ 1.10 × 10¹⁵ 1.18 × 10¹⁵ 3.02 × 10¹⁵ F_(μ BL) _(—) _(GΔ β) 6.15 4.28 2.61 2.25 11.25 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹²) Sandpaper tension winding 0.85 0.46 0.3 0.22 0.98 loss increase (dB/km)

TABLE 2 Example 6 7 8 9 10 Outside diameter of 80 125 125 80 80 glass portion(μm) Outside diameter of 115 159 190 113 113 primary coating layer(μm) Outside diameter of 153 193 240 164 153 secondary coating layer(μm) Young's modulus of 74 74 74 114 74 glass portion(GPa) Young's modulus of 0.2 0.5 0.6 0.2 0.2 primary coating layer(MPa) Young's modulus of 1150 1150 800 1150 1150 secondary coating layer(MPa) Thickness of primary 17.5 17 32.5 16.5 16.5 coating layer(μm) Thickness of secondary 19 17 25 25.5 20 coating layer(μm) Coating thickness(μm) 36.5 34 57.5 42 36.5 Bending rigidity of 1.49 × 10¹¹ 8.87 × 10¹¹ 8.87 × 10¹¹ 2.29 × 10¹¹ 1.49 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 2.11 × 10¹⁰ 4.22 × 10¹⁰ 7.91 × 10¹⁰ 3.16 × 10¹⁰ 2.17 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 κ_(s)(MPa) 0.91 3.68 2.31 0.97 0.97 Deformation resistance of 17.82 6.79 7.83 34.78 20.75 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 4.53 1.92 0.48 1.29 4.72 10⁻²⁷) Mode field diameter(μm) 8.48 8.5 8.55 8.51 8.47 Cable cutoff wavelength(μm) 1.209 1.203 1.197 1.287 1.330 MAC value(a.u.) 7.01 7.07 7.14 6.61 6.37 Macrobend loss(dB/turn) 0.200 0.091 0.133 0.013 0.035 Propagation constant 11403 11971 11623 14259 14296 difference (rad/m) Zero dispersion 1.313 1.313 1.313 1.309 1.309 wavelength(μm) Zero dispersion 0.085 0.086 0.086 0.091 0.091 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/(rad/μm)⁸) 3.50 × 10¹⁵ 2.37 × 10¹⁵ 3.00 × 10¹⁵ 5.85 × 10¹⁴ 5.73 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 15.83 4.54 1.45 0.76 2.70 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹²) Sandpaper tension winding 1.5 0.3 0.05 0.218 0.315 loss increase (dB/km)

TABLE 3 Example 11 12 13 14 15 Outside diameter of 80 80 80 80 90 glass portion(μm) Outside diameter of 114 114 115 115 121 primary coating layer(μm) Outside diameter of 153 164 153 153 159 secondary coating layer(μm) Young's modulus of 74 74 74 74 74 glass portion(GPa) Young's modulus of 0.2 0.2 0.2 0.2 0.2 primary coating layer(MPa) Young's modulus of 1150 1150 1150 1400 1150 secondary coating layer(MPa) Thickness of primary 17 17 17.5 17.5 15.5 coating layer(μm) Thickness of secondary 19.5 25 19 19 19 coating layer(μm) Coating thickness(μm) 36.5 42 36.5 36.5 34.5 Bending rigidity of 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 2.38 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 2.14 × 10¹⁰ 3.13 × 10¹⁰ 2.11 × 10¹⁰ 2.56 × 10¹⁰ 2.40 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 κ_(s)(MPa) 0.94 0.94 0.91 0.91 1.16 Deformation resistance of 19.25 32.79 17.82 21.65 15.90 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 4.61 2.98 4.53 3.72 2.74 10⁻²⁷) Mode field diameter(μm) 8.47 8.5 8.36 8.4 8.52 Cable cutoff wavelength(μm) 1.357 1.367 1.174 1.188 1.221 MAC value(a.u.) 6.24 6.22 7.12 7.07 6.98 Macrobend loss(dB/turn) 0.010 0.010 0.080 0.040 0.040 Propagation constant 15223 15007 13198 15278 14687 difference (rad/m) Zero dispersion 1.309 1.309 1.310 1.310 1.309 wavelength(μm) Zero dispersion 0.091 0.091 0.089 0.089 0.091 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 3.47 × 10¹⁴ 3.89 × 10¹⁴ 1.09 × 10¹⁵ 3.37 × 10¹⁴ 4.62 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 1.60 1.16 4.92 1.25 1.26 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹² Sandpaper tension winding 0.231 0.289 0.57 0.27 0.197 loss increase (dB/km)

TABLE 4 Example 16 17 18 19 20 Outside diameter of 90 90 80 80 80 glass portion(μm) Outside diameter of 121 121 115 115 115 primary coating layer(μm) Outside diameter of 159 159 153 164 153 secondary coating layer(μm) Young's modulus of 74 74 74 74 74 glass portion(GPa) Young's modulus of 0.2 0.2 0.2 0.2 0.2 primary coating layer(MPa) Young's modulus of 1150 1150 1150 1150 1150 secondary coating layer(MPa) Thickness of primary 15.5 15.5 17.5 17.5 17.5 coating layer(μm) Thickness of secondary 19 19 19 24.5 19 coating layer(μm) Coating thickness(μm) 34.5 34.5 36.5 42 36.5 Bending rigidity of 2.38 × 10¹¹ 2.38 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 2.40 × 10¹⁰ 2.40 × 10¹⁰ 2.11 × 10¹⁰ 3.10 × 10¹⁰ 2.11 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 κ_(s)(MPa) 1.16 1.16 0.91 0.91 0.91 Deformation resistance of 15.90 15.90 17.82 30.87 17.82 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 2.74 2.74 4.53 2.90 4.53 10⁻²⁷) Mode field diameter(μm) 8.506 8.46 7.645 7.64 7.607 Cable cutoff wavelength(μm) 1.270 1.286 1.184 1.245 1.271 MAC value(a.u.) 6.70 6.58 6.46 6.14 5.99 Macrobend loss(dB/turn) 0.017 0.008 0.070 0.006 0.004 Propagation constant 14392 15187 12929 13865 14825 difference (rad/m) Zero dispersion 1.309 1.309 1.336 1.336 1.336 wavelength(μm) Zero dispersion 0.091 0.091 0.079 0.079 0.079 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 5.43 × 10¹⁴ 3.53 × 10¹⁴ 1.28 × 10¹⁵ 7.32 × 10¹⁴ 4.29 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 1.49 0.97 5.80 2.12 1.94 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹² Sandpaper tension winding 0.232 0.164 0.577 0.23 0.259 loss increase (dB/km)

TABLE 5 Example 21 22 Outside diameter of 80 80 glass portion(μm) Outside diameter of 115 115 primary coating layer(μm) Outside diameter of 153 153 secondary coating layer(μm) Young's modulus of 74 74 glass portion(GPa) Young's modulus of 0.2 0.2 primary coating layer(MPa) Young's modulus of 1150 1400 secondary coating layer(MPa) Thickness of primary 17.5 17.5 coating layer(μm) Thickness of secondary 19 19 coating layer(μm) Coating thickness(μm) 36.5 36.5 Bending rigidity of 1.49 × 10¹¹ 1.49 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 2.11 × 10¹⁰ 2.56 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 κ_(s)(MPa) 0.91 0.91 Deformation resistance of 17.82 21.65 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 4.53 3.72 10⁻²⁷) Mode field diameter(μm) 7.627 7.7 Cable cutoff wavelength(μm) 1.300 1.183 MAC value(a.u.) 5.87 6.51 Macrobend loss(dB/turn) 0.001 0.040 Propagation constant 15702 13192 difference (rad/m) Zero dispersion 1.336 1.339 wavelength(μm) Zero dispersion 0.079 0.079 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 2.71 × 10¹⁴ 1.09 × 10¹⁵ F_(μ BL) _(—) _(GΔ β) 1.22 4.06 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹² Sandpaper tension winding 0.165 0.334 loss increase (dB/km)

The present inventor plotted the values of the microbend loss characteristic factor F_(μBL_GΔβ) and the values of the sandpaper tension winding loss increase F_(μBL_GΔβ) of each of Samples 1 to 22 with respect to a coordinate system in which the value of microbend loss characteristic factor F_(μBL_GΔβ) is on the horizontal axis (X-axis) and the value of sandpaper tension winding loss increase α_(μBL) is on the vertical axis (Y-axis). As a result, the scatter diagram shown in FIG. 5 was obtained. When the function was obtained from this scatter diagram using the least-squares method, a linear function with a positive slope represented by the formula (7) below was obtained. Furthermore, the correlation coefficient of the data in FIG. 5 was 94% or more. Y=0.0986X  (7)

That is, it was found that the value of the microbend loss characteristic factor F_(μBL_GΔβ) and the value of the sandpaper tension winding loss increase α_(μBL) had a high correlation, and specifically the value of the microbend loss characteristic factor F_(μBL_GΔβ) and the value of the sandpaper tension winding loss increase α_(μBL) had a proportional relationship having a generally positive slope.

By the way, as described above, the tape slot type cable (RSCC) has the required characteristics that the value of the sandpaper tension winding loss increase α_(μBL) is 0.60 (dm/km) or less. Furthermore, the small-diameter high-density cable (UHDC) has the required characteristics that the value of the sandpaper tension winding loss increase α_(μBL) is 0.34 (dm/km) or less. Therefore, Table 6 below indicates the values of the microbend loss characteristic factor F_(μBL_GΔβ), the values of the sandpaper tension winding loss increase α_(μBL), the pass/fail of the required characteristics of the tape slot type cable (RSCC), and the pass/fail of the required characteristics of the small-diameter high-density cable (UHDC) of Examples 1 to 22. Note that in Table 6, Y means that the required characteristics are satisfied, and N means that the required characteristics are not satisfied.

TABLE 6 F_(μ BL) _(—) _(GΔ β) α μBL ([GPa⁻¹ · μm^(−2.5)/ Application to RSCC Application to UHDC Example (dB/km) rad⁸] × 10⁻¹²) (F_(μ BL) _(—) _(GΔ β) ≤ 6.1) (F_(μ BL) _(—) _(GΔ β) ≤ 4.1) 9 0.22 0.76 Y Y 17 0.16 0.97 Y Y 12 0.29 1.16 Y Y 21 0.17 1.22 Y Y 14 0.27 1.25 Y Y 15 0.27 1.26 Y Y 8 0.05 1.45 Y Y 16 0.23 1.49 Y Y 11 0.23 1.60 Y Y 20 0.26 1.94 Y Y 19 0.23 2.12 Y Y 4 0.22 2.25 Y Y 3 0.30 2.61 Y Y 10 0.32 2.70 Y Y 22 0.33 4.06 Y Y 2 0.46 4.28 Y N 7 0.30 4.54 Y Y 13 0.57 4.92 Y N 18 0.58 5.80 Y N 1 0.85 6.15 N N 5 0.98 11.25 N N 6 1.50 15.83 N N

From Table 6, it was found that when the value of the microbend loss characteristic factor F_(μBL_GΔβ) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, the value of the sandpaper tension winding loss increase α_(μBL) tends to be approximately 0.60 or less and when the value of the microbend loss characteristic factor F_(μBL_GΔβ) is larger than 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²), the value of the sandpaper tension winding loss increase α_(μBL) tends to exceed 0.60. In other words, it was found that the required characteristics of the tape slot type cable can be satisfied by adjusting the values of the parameters described in Tables 1 to 5 described above so that the value of the microbend loss characteristic factor F_(μBL_GΔβ) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.

Furthermore, it was found that when the value of the microbend loss characteristic factor F_(μBL_GΔβ) is 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, the value of the sandpaper tension winding loss increase α_(μBL) tends to be approximately 0.34 or less and when the value of the microbend loss characteristic factor F_(μBL_GΔβ) is larger than 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²), the value of the sandpaper tension winding loss increase α_(μBL) tends to exceed 0.34. In other words, it was found that in addition to the required characteristics of the tape slot type cable, the required characteristics of the small-diameter high-density cable can be satisfied by adjusting the values of the parameters described in Tables 1 to 5 described above so that the value of the microbend loss characteristic factor F_(μBL_GΔβ) is 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.

Specifically, among Samples 1 to 22, the samples satisfying the required characteristics of the tape slot type cable were the samples excluding Samples 1, 5, and 6. Furthermore, the samples satisfying the required characteristics of the small-diameter high-density cable in addition to the required characteristics of the tape slot type cable were the samples excluding Examples 1, 2, 5, 6, 13, and 18.

Furthermore, among the samples satisfying at least the required characteristics of the tape slot type cable among Samples 1 to 22, the samples excluding Samples 4, 7, and 8 have an outside diameter of the glass portion of 80 μm or 90 μm smaller than the outside diameter (125 μm) of the glass portion of the general optical fiber. Specifically, Samples 2, 3, 9 to 14, and 18 to 22 have an outside diameter of the glass portion of 80 μm, and Samples 15 to 17 have an outside diameter of the glass portion of 90 μm. That is, it was found that, by adjusting the parameters as in Samples 2, 3, and 9 to 22, an optical fiber that satisfies at least the required characteristics of the tape slot type cable and has the outside diameter of the glass portion smaller than that of the general optical fiber can be formed.

Furthermore, it was found that the samples, excluding Sample 8, satisfying at least the required characteristics of the tape slot type cable among Samples 1 to 22 have a coating thickness smaller than the coating thickness (approximately 60 μm) of the general optical fiber. Specifically, it was found that Samples 3, 9, and 12 have a coating thickness of 42.0 μm, Samples 10, 11, 13, 14, 18, and 20 to 22 have a coating thickness of 36.5 μm, Sample 2 has a coating thickness of 36.0 μm, Samples 15 to 17 have a coating thickness of 34.5 μm, and Samples 4 and 7 have a coating thickness of 34.0 μm. That is, it was found that, by adjusting the parameters as in Samples 2 to 4, 7, and 9 to 22, an optical fiber that satisfies at least the required characteristics of the tape slot type cable and has the coating thickness smaller than that of the general optical fiber can be formed.

As described above, it was found that, among Samples 1 to 22, the samples excluding Samples 1, 5, 6, and 8 satisfy at least the required characteristics of the tape slot type cable and have the outside diameter of the glass portion and the coating thickness smaller than those of the general optical fiber. By forming both the outside diameter of the glass portion and the coating thickness to be smaller than the outside diameter of the glass portion and the coating thickness of the general optical fiber, it is possible to effectively realize a reduction in diameter of the optical fiber.

Furthermore, the optical fibers of Samples 1 to 22 have an MFD of 7.6 μm or more. When the MFD is too small, an MFD mismatch can occur when connection to a general-purpose optical fiber is established. However, when the MFD of the optical fiber is 7.6 μm or more, the MFD mismatch when connection to a general-purpose optical fiber is established can be small. Therefore, the occurrence of connection loss can be effectively suppressed.

Moreover, the optical fibers of Samples 5 to 8 meet the international standard ITU-.G.657.A1. That is, the MFD at a wavelength of 1310 nm is 8.2 μm or more and 9.6 μm or less, the cable cutoff wavelength is 1260 nm or less, the zero dispersion wavelength is 1300 nm or more and 1324 nm or less, the zero dispersion slope is 0.073 ps/km/nm² or more and 0.092 ps/km/nm² or less, and the macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm is 1.5 dB/turn or less. Furthermore, the optical fibers of Samples 1 to 4 satisfy ITU-T.G.657.A2. That is, the MFD at a wavelength of 1310 nm is 8.2 μm or more and 9.6 μm or less, the cable cutoff wavelength is 1260 nm or less, the zero dispersion wavelength is 1300 nm or more and 1324 nm or less, the zero dispersion slope is 0.073 ps/km/nm² or more and 0.092 ps/km/nm² or less, and the macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm is 0.2 dB/turn or less. Furthermore, the optical fibers of Samples 13 to 15 satisfy ITU-T.G.657.B3. That is, the MFD at a wavelength of 1310 nm is 8.26 μm or more and 9.6 μm or less, the cable cutoff wavelength is 1260 nm or less, the zero dispersion wavelength is 1300 nm or more and 1324 nm, the zero dispersion slope is 0.073 ps/km/nm² or more and 0.092 ps/km/nm² or less, and the macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm is 0.1 dB/turn or less.

EXAMPLES 23 TO 28

Furthermore, the present inventor determined the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 23 to 28 of optical fiber adjusted as indicated in Table 7 below on the assumption of an optical fiber having the same optical characteristics as Samples 16, 17, and 19, specifically, the same MFD, cable cutoff wavelength, MAC value, macrobend loss (bending loss), propagation constant difference, zero dispersion wavelength, and zero dispersion slope as those samples, the same thickness of the primary coating layer and thickness of the secondary coating layer as those of Sample 19, and an outside diameter of the glass portion of 65 μm.

TABLE 7 Example 23 24 25 26 27 28 Outside diameter of 65 65 65 65 65 65 glass portion(μm) Outside diameter of 100 100 100 100 100 100 primary coating layer(μm) Outside diameter of 149 149 149 149 149 149 secondary coating layer(μm) Young's modulus of 74 74 74 74 74 74 glass portion(GPa) Young's modulus of 0.2 0.2 0.2 0.2 0.2 0.2 primary coating layer(MPa) Young's modulus of 1400 1150 1400 1150 1400 1150 secondary coating layer(MPa) Thickness of primary 17.5 17.5 17.5 17.5 17.5 17.5 coating layer(μm) Thickness of secondary 24.5 24.5 24.5 24.5 24.5 24.5 coating layer(μm) Coating thickness(μm) 42 42 42 42 42 42 Bending rigidity of 6.48 × 10¹⁰ 6.48 × 10¹⁰ 6.48 × 10¹⁰ 6.48 × 10¹⁰ 6.48 × 10¹⁰ 6.48 × 10¹⁰ glass portion(MPa · μm⁴) Bending rigidity of 2.70 × 10¹⁰ 2.22 × 10¹⁰ 2.70 × 10¹⁰ 2.22 × 10¹⁰ 2.70 × 10¹⁰ 2.22 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 3 κ_(s)(MPa) 0.74 0.74 0.74 0.74 0.56 0.56 Deformation resistance of 49.99 41.10 49.99 41.10 49.94 41.05 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 9.15 11.14 9.15 11.14 5.15 6.27 10⁻²⁷) Mode field diameter(μm) 7.64 7.64 8.506 8.506 8.46 8.46 Cable cutoff wavelength(μm) 1.245 1.245 1.270 1.270 1.286 1.286 MAC value(a.u.) 6.14 6.14 6.70 6.70 6.58 6.58 Macrobend loss(dB/turn) 0.006 0.006 0.017 0.017 0.008 0.008 Propagation constant 13865 13865 14392 14392 15187 15187 difference (rad/m) Zero dispersion 1.336 1.336 1.309 1.309 1.309 1.309 wavelength(μm) Zero dispersion 0.079 0.079 0.091 0.091 0.091 0.091 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 7.32 × 10¹⁴ 7.32 × 10¹⁴ 5.43 × 10¹⁴ 5.43 × 10¹⁴ 3.53 × 10¹⁴ 3.53 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 6.70 8.15 4.97 6.05 1.82 2.21 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹²) Application to RSCC N N Y Y Y Y Application to UHDC N N N N Y Y

As indicated in Table 7, each of Samples 23 to 28 has an outside diameter of the glass portion of 65 μm and a coating thickness of 42 μm. It was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 25 to 28 are all 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 25 to 28 satisfy the required characteristics of the tape slot type cable. Furthermore, it was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 27 and 28 are 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 27 and 28 satisfy the required characteristics of the small-diameter high-density cable in addition to the required characteristics of the tape slot type cable. Note that similar to Samples 25 to 28, Samples 23 and 24 have an outside diameter of the glass portion of 65 μm and a thickness of the coating layer of 42 μm, but the values of the microbend loss characteristic factor F_(μBL_GΔβ) exceed 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²), and do not satisfy the required characteristics of the tape slot type cable or the required characteristics of the small-diameter high-density cable.

Examples 29 to 36

Furthermore, the present inventor determined the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 29 to 36 of optical fiber adjusted as indicated in Tables 8 and 9 on the assumption of an optical fiber having the same optical characteristics as Samples 15, 16, 17, and 19, specifically, the same MFD, cable cutoff wavelength, MAC value, macrobend loss, propagation constant difference, zero dispersion wavelength, and zero dispersion slope as those samples, the same thickness of the primary coating layer and thickness of the secondary coating layer as those of Sample 19, and an outside diameter of the glass portion of 70 μm.

TABLE 8 Example 29 30 31 32 Outside diameter of 70 70 70 70 glass portion(μm) Outside diameter of 105 105 105 105 primary coating layer(μm) Outside diameter of 154 154 154 154 secondary coating layer(μm) Young's modulus of 74 74 74 74 glass portion(GPa) Young's modulus of 0.2 0.2 0.2 0.2 primary coating layer(MPa) Young's modulus of 1400 1150 1400 1150 secondary coating layer(MPa) Thickness of primary 17.5 17.5 17.5 17.5 coating layer(μm) Thickness of secondary 24.5 24.5 24.5 24.5 coating layer(μm) Coating thickness(μm) 42 42 42 42 Bending rigidity of 8.72 × 10¹⁰ 8.72 × 10¹⁰ 8.72 × 10¹⁰ 8.72 × 10¹⁰ glass portion(MPa · μm⁴) Bending rigidity of 3.03 × 10¹⁰ 2.49 × 10¹⁰ 3.03 × 10¹⁰ 2.49 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 κ_(s)(MPa) 0.80 0.80 0.80 0.80 Deformation resistance of 45.30 37.24 45.30 37.24 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 5.66 6.89 5.66 6.89 10⁻²⁷) Mode field diameter(μm) 7.64 7.64 8.52 8.52 Cable cutoff wavelength(μm) 1.245 1.245 1.221 1.221 MAC value(a.u.) 6.14 6.14 6.98 6.98 Macrobend loss(dB/turn) 0.006 0.006 0.040 0.040 Propagation constant 13865 13865 14687 14687 difference (rad/m) Zero dispersion 1.336 1.336 1.309 1.309 wavelength(μm) Zero dispersion 0.079 0.079 0.091 0.091 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 7.32 × 10¹⁴ 7.32 × 10¹⁴ 4.62 × 10¹⁴ 4.62 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 4.15 5.05 2.62 3.18 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹² Application to RSCC Y Y Y Y Application to UHDC N N Y Y

TABLE 9 Example 33 34 35 36 Outside diameter of 70 70 70 70 glass portion(μm) Outside diameter of 105 105 105 105 primary coating layer(μm) Outside diameter of 154 154 154 154 secondary coating layer(μm) Young's modulus of 74 74 74 74 glass portion(GPa) Young's modulus of 0.2 0.2 0.2 0.2 primary coating layer(MPa) Young's modulus of 1400 1150 1400 1150 secondary coating layer(MPa) Thickness of primary 17.5 17.5 17.5 17.5 coating layer(μm) Thickness of secondary 24.5 24.5 24.5 24.5 coating layer(μm) Coating thickness(μm) 42 42 42 42 Bending rigidity of 8.72 × 10¹⁰ 8.72 × 10¹⁰ 8.72 × 10¹⁰ 8.72 × 10¹⁰ glass portion(MPa · μm⁴) Bending rigidity of 3.03 × 10¹⁰ 2.49 × 10¹⁰ 3.03 × 10¹⁰ 2.49 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 κ_(s)(MPa) 0.80 0.80 0.80 0.80 Deformation resistance of 45.30 37.24 45.30 37.24 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 5.66 6.89 5.66 6.89 10⁻²⁷) Mode field diameter(μm) 8.506 8.506 8.46 8.46 Cable cutoff wavelength(μm) 1.270 1.270 1.286 1.286 MAC value(a.u.) 6.70 6.70 6.58 6.58 Macrobend loss(dB/turn) 0.017 0.017 0.008 0.008 Propagation constant 14392 14392 15187 15187 difference (rad/m) Zero dispersion 1.309 1.309 1.309 1.309 wavelength(μm) Zero dispersion 0.091 0.091 0.091 0.091 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 5.43 × 10¹⁴ 5.43 × 10¹⁴ 3.53 × 10¹⁴ 3.53 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 3.08 3.75 2.00 2.44 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹² Application to RSCC Y Y Y Y Application to UHDC Y Y Y Y

As indicated in Tables 8 and 9, each of Samples 29 to 36 has an outside diameter of the glass portion of 70 μm and a coating thickness of 42 μm. It was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 26 to 28 are all 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 29 to 36 satisfy the required characteristics of the tape slot type cable. Furthermore, it was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 31, 33, 35 and 36 are all 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 31, 33, 35 and 36 satisfy the required characteristics of the small-diameter high-density cable in addition to the required characteristics of the tape slot type cable.

Examples 37 to 42

Furthermore, the present inventor determined the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 37 to 42 of optical fiber adjusted as indicated in Table 10 below on the assumption of an optical fiber having the same optical characteristics as Samples 15, 17, and 19, specifically, the same MFD, cable cutoff wavelength, MAC value, macrobend loss, propagation constant difference, zero dispersion wavelength, and zero dispersion slope as those samples, the same thickness of the primary coating layer and thickness of the secondary coating layer as those of Sample 19, and an outside diameter of the glass portion of 75 μm.

TABLE 10 Example 37 38 39 40 41 42 Outside diameter of 75 75 75 75 75 75 glass portion(μm) Outside diameter of 110 110 110 110 110 110 primary coating layer(μm) Outside diameter of 159 159 159 159 159 159 secondary coating layer(μm) Young's modulus of 74 74 74 74 74 74 glass portion(GPa) Young's modulus of 0.15 0.15 0.2 0.2 0.2 0.2 primary coating layer(MPa) Young's modulus of 1400 1150 1400 1150 1400 1150 secondary coating layer(MPa) Thickness of primary 17.5 17.5 17.5 17.5 17.5 17.5 coating layer(μm) Thickness of secondary 24.5 24.5 24.5 24.5 24.5 24.5 coating layer(μm) Coating thickness(μm) 42 42 42 42 42 42 Bending rigidity of 1.15 × 10¹¹ 1.15 × 10¹¹ 1.15 × 10¹¹ 1.15 × 10¹¹ 1.15 × 10¹¹ 1.15 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 3.39 × 10¹⁰ 2.78 × 10¹⁰ 3.39 × 10¹⁰ 2.78 × 10¹⁰ 3.39 × 10¹⁰ 2.78 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 3 κ_(s)(MPa) 0.64 0.64 0.86 0.86 0.86 0.86 Deformation resistance of 41.13 33.81 41.18 33.86 41.18 33.86 secondary coating layer(MPa) F_(μBL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 2.04 2.48 3.62 4.40 3.62 4.40 10⁻²⁷) Mode field diameter(μm) 7.64 7.64 8.52 8.52 8.46 8.46 Cable cutoff wavelength(μm) 1.245 1.245 1.221 1.221 1.286 1.286 MAC value(a.u.) 6.14 6.14 6.98 6.98 6.58 6.58 Macrobend loss(dB/turn) 0.006 0.006 0.040 0.040 0.008 0.008 Propagation constant 13865 13865 14687 14687 15187 15187 difference (rad/m) Zero dispersion 1.336 1.336 1.309 1.309 1.309 1.309 wavelength(μm) Zero dispersion 0.079 0.079 0.091 0.091 0.091 0.091 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 7.32 × 10¹⁴ 7.32 × 10¹⁴ 7.32 × 10¹⁴ 4.62 × 10¹⁴ 3.53 × 10¹⁴ 3.3 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 1.49 1.82 1.67 2.03 1.28 1.56 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹²) Application to RSCC Y Y Y Y Y Y Application to UHDC Y Y Y Y Y Y

As indicated in Table 10, each of Samples 37 to 42 has an outside diameter of the glass portion of 75 μm and a coating thickness of 42 μm. It was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 37 to 42 are all 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 37 to 42 satisfy the required characteristics of the small-diameter high-density cable in addition to the required characteristics of the tape slot type cable.

Examples 43 to 48

Furthermore, the present inventor determined the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 43 to 48 of optical fiber adjusted as indicated in Table 11 below on the assumption of an optical fiber having the same optical characteristics as Samples 15, 17, and 19, specifically, the same MFD, cable cutoff wavelength, MAC value, macrobend loss, propagation constant difference, zero dispersion wavelength, and zero dispersion slope as those samples, the same thickness of the primary coating layer and thickness of the secondary coating layer as those of Sample 19, and an outside diameter of the glass portion of 80 μm.

TABLE 11 Example 43 44 45 46 47 48 Outside diameter of 80 80 80 80 80 80 glass portion(μm) Outside diameter of 100 100 100 100 100 100 primary coating layer(μm) Outside diameter of 125 125 125 125 125 125 secondary coating layer(μm) Young's modulus of 74 74 74 74 74 74 glass portion(GPa) Young's modulus of 0.12 0.12 0.12 0.12 0.12 0.12 primary coating layer(MPa) Young's modulus of 1400 1150 1400 1150 1400 1150 secondary coating layer(MPa) Thickness of primary 12.5 12.5 12.5 12.5 12.5 12.5 coating layer(μm) Thickness of secondary 10 10 10 10 10 10 coating layer(μm) Coating thickness(μm) 22.5 22.5 22.5 22.5 22.5 22.5 Bending rigidity of 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 8.42 × 10⁹  6.92 × 10⁹  8.42 × 10⁹  6.92 × 10⁹  8.42 × 10⁹  6.92 × 10⁹  secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 3 κ_(s)(MPa) 0.77 0.77 0.77 0.77 0.77 0.77 Deformation resistance of 5.85 4.83 5.85 4.83 5.85 4.83 secondary coating layer(MPa) F_(μBL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 8.60 10.45 8.60 10.45 8.60 10.45 10⁻²⁷) Mode field diameter(μm) 7.64 7.64 8.52 8.52 8.46 8.46 Cable cutoff wavelength(μm) 1.245 1.245 1.221 1.221 1.286 1.286 MAC value(a.u.) 6.14 6.14 6.98 6.98 6.58 6.58 Macrobend loss(dB/turn) 0.006 0.006 0.040 0.040 0.008 0.008 Propagation constant 13865 13865 14687 14687 15187 15187 difference(rad/m) Zero dispersion 1.336 1.336 1.309 1.309 1.309 1.309 wavelength(μm) Zero dispersion 0.079 0.079 0.091 0.091 0.091 0.091 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 7.32 × 10¹⁴ 7.32 × 10¹⁴ 4.62 × 10¹⁴ 4.62 × 10¹⁴ 3.53 × 10¹⁴ 3.53 × 10¹⁴ F_(μ BL) _(—) _(GΔ β) 6.29 7.65 3.97 4.82 3.04 3.69 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹²) Application to RSCC N N Y Y Y Y Application to UHDC N N Y N Y Y

As indicated in Table 11, each of Samples 45 to 48 has an outside diameter of the glass portion of 80 μm, an outside diameter of the secondary coating layer of 125 μm, and a thickness of the coating layer of 22.5 μm. It was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 45 to 48 are all 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 45 to 48 satisfy the required characteristics of the tape slot type cable. Furthermore, it was found that the value of the microbend loss characteristic factor F_(μBL_GΔβ) of Sample 47 is 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Sample 47 satisfies the required characteristics of the small-diameter high-density cable in addition to the required characteristics of the tape slot type cable. Note that similar to Samples 45 to 48, Samples 43 and 44 have an outside diameter of the glass portion of 80 μm, an outside diameter of the secondary coating layer of 125 μm, and a thickness of the coating layer of 22.5 μm, but the values of the microbend loss characteristic factor F_(μBL_GΔβ) exceed 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²), and do not satisfy the required characteristics of the tape slot type cable or the required characteristics of the small-diameter high-density cable.

Examples 49 to 57

The present inventor determined the value of the microbend loss characteristic factor F_(μBL_GΔβ) of the optical fiber samples 49 to 57 adjusted as shown in Tables 12 and 13.

TABLE 12 Example 49 50 51 52 53 Outside diameter of 80 80 80 125 125 glass portion(μm) Outside diameter of 125 115 115 151 159 primary coating layer(μm) Outside diameter of 156 153 153 184 193 secondary coating layer(μm) Young's modulus of 74 74 74 74 74 glass portion(GPa) Young's modulus of 0.20 0.18 0.08 0.18 0.18 primary coating layer(MPa) Young's modulus of 1150 1250 1250 1250 1250 secondary coating layer(MPa) Thickness of primary 22.5 17.5 17.5 13.0 17.0 coating layer(μm) Thickness of secondary 15.5 19.0 19.0 16.5 17.0 coating layer(μm) Coating thickness(μm) 38.0 36.5 36.5 29.5 34 Bending rigidity of 1.49 × 10¹¹ 1.49 × 10¹¹ 1.49 × 10¹¹ 8.87 × 10¹¹ 8.87 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 1.97 × 10¹⁰ 2.29 × 10¹⁰ 2.29 × 10¹⁰ 3.84 × 10¹⁰ 4.59 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 3 κ_(s)(MPa) 0.71 0.82 0.37 1.73 1.32 Deformation resistance of 9.22 19.33 19.23 7.39 7.01 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 3.66 3.38 0.67 0.44 0.23 10⁻²⁷) Mode field diameter(μm) 8.39 8.58 8.4 8.52 8.6 Cable cutoff wavelength(μm) 1.230 1.220 1.230 1.210 1.220 MAC value(a.u.) 6.84 7.06 6.83 7.06 7.04 Macrobend loss(dB/turn) 0.03 0.01 0.03 0.20 0.14 Propagation constant 13106 13249 13154 11363 12083 difference (rad/m) Zero dispersion 1.313 1.313 1.313 1.312 1.312 wavelength(μm) Zero dispersion 0.086 0.086 0.086 0.087 0.087 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 1.15 × 10¹⁵ 1.05 × 10¹⁵ 1.12 × 10¹⁵ 3.60 × 10¹⁵ 2.20 × 10¹⁵ F_(μ BL) _(—) _(GΔ β) 4.21 3.56 0.75 1.57 0.51 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹² Sandpaper tension 0.46 0.38 0.15 0.20 0.30 winding loss increase(dB/km) Application to RSCC Y Y Y Y Y Application to UHDC N Y Y Y Y

TABLE 13 Example 54 55 56 57 Outside diameter of 125 125 125 125 glass portion(μm) Outside diameter of 151 159 190 190 primary coating layer(μm) Outside diameter of 184 193 239 239 secondary coating layer(μm) Young's modulus of 74 74 74 74 glass portion(GPa) Young's modulus of 0.08 0.45 0.70 0.70 primary coating layer(MPa) Young's modulus of 1250 1250 850 850 secondary coating layer(MPa) Thickness of primary 13.0 17.0 32.5 32.5 coating layer(μm) Thickness of secondary 16.5 17.0 24.5 24.5 coating layer(μm) Coating thickness(μm) 29.5 34.0 57.0 57.0 Bending rigidity of 8.87 × 10¹¹ 8.87 × 10¹¹ 8.87 × 10¹¹ 8.87 × 10¹¹ glass portion(MPa · μm⁴) Bending rigidity of 3.84 × 10¹⁰ 4.59 × 10¹⁰ 8.18 × 10¹⁰ 8.18 × 10¹⁰ secondary coating layer(MPa · μm⁴) μ (a.u.) 3 3 3 3 κ_(s)(MPa) 0.77 3.31 2.69 2.69 Deformation resistance of 7.29 7.28 8.03 8.03 secondary coating layer(MPa) F_(μ BL) _(—) _(G)(GPa⁻¹ · μm^(−10.5) · 0.09 1.43 0.64 0.64 10⁻²⁷) Mode field diameter(μm) 8.45 8.49 8.54 9.12 Cable cutoff wavelength(μm) 1.200 1.210 1.220 1.210 MAC value(a.u.) 7.04 7.02 6.98 7.57 Macrobend loss(dB/turn) 0.20 0.22 0.06 0.30 Propagation constant 11323 11269 12782 10446 difference (rad/m) Zero dispersion 1.312 1.313 1.313 1.313 wavelength(μm) Zero dispersion 0.087 0.087 0.086 0.086 slope(ps/km/nm²) F_(μ BL) _(—) _(Δ β)(1/rad/μm)⁸) 3.70 × 10¹⁵ 3.85 × 10¹⁵ 1.40 × 10¹⁵ 7.05 × 10¹⁵ F_(μ BL) _(—) _(GΔ β) 0.32 5.51 0.90 4.50 ([GPa⁻¹ · μm^(−2.5)/rad⁸] × 10⁻¹² Sandpaper tension 0.10 0.40 0.10 0.20 winding loss increase(dB/km) Application to RSCC Y Y Y Y Application to UHDC Y N Y N

As indicated in Table 12, 13, it was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples to 57 are all 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 49 to 57 satisfy the required characteristics of the tape slot type cable. Furthermore, it was found that the values of the microbend loss characteristic factor F_(μBL_GΔβ) of Samples 50 to 54, and 56 are all 4.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less, and Samples 49 to 54, and 56 satisfy the required characteristics of the small-diameter high-density cable in addition to the required characteristics of the tape slot type cable.

FIG. 6 is a diagram showing the relationship between the value of the microbend loss characteristic factor and sandpaper tension winding loss increase in the optical fibers shown in the samples 1 to 22 and 49 to 57. When the function was obtained from this scatter diagram using the least-squares method, a linear function with a positive slope represented by the formula (8) below was obtained. Furthermore, the correlation coefficient of the data in FIG. 6 was 85% or more. Y=0.0963X  (8)

When the mode field diameter is 9.12 μm or less, the effect of confining light is further improved. Therefore, if the optical fiber is applied to the tape slot type cable or the small-diameter high-density cable and the accommodation density of the tape core wire in the slot is increased, microbend loss can be reduced when the lateral pressure applied to the optical fiber becomes large. When the mode field diameter is less than 8.6 μm, the effect of confining light is further improved. Therefore, if the optical fiber is applied to the tape slot type cable or the small-diameter high-density cable and the accommodation density of the tape core wire in the slot is increased, microbend loss can be further reduced when the lateral pressure applied to the optical fiber becomes large.

There are situations in which the tape core wire has a high density and the lateral pressure received by the optical fiber constituting the tape core wire becomes particularly large. Even in this situation, if the value of the microbend loss characteristic factor is less than 5.83, the microbend loss can be suppressed to the extent that it can be applied to the tape slot type cable having a strict requirement for microbend loss. Further, the tape core wire has a higher density, and the lateral pressure received by the optical fiber constituting the tape core wire is further increased. Even in this situation, if the value of the microbend loss characteristic factor is 5.8 or less, the microbend loss can be suppressed to the extent that it can be applied to the tape slot type cable having a strict requirement for microbend loss. Further, the tape core wire has a very high density, and the lateral pressure received by the optical fiber constituting the tape core wire becomes very large. Even in this situation, if the value of the microbend loss characteristic factor is 3.75 or less, the microbend loss can be suppressed to the extent that it can be applied to the tape slot type cable or the small-diameter high-density cable having a strict requirement for microbend loss.

Since the mode field diameter is 7.6 μm or more, connection loss can be suppressed even when connected to an optical fiber having a large mode field diameter.

Although the present invention has been described above by taking the aforementioned embodiments as an example, the present invention is not limited thereto.

For example, in the first and second embodiments described above, the example in which the secondary coating layer is the outermost layer of the optical fiber has been described. However, even when a colored layer is further provided as a third coating layer on the outer periphery of the secondary coating layer, the secondary coating layer and the colored layer can be applied to the present invention as a second coating layer, i.e., the secondary coating layer as long as the Young's modulus of the colored layer is not significantly different from the Young's modulus of the secondary coating layer.

According to one or more embodiments of the present invention, an optical fiber capable of suppressing microbend loss is provided, and can be used in a field such as a communication infrastructure. 

What is claimed is:
 1. An optical fiber comprising: a glass portion including a core; a clad surrounding the core; a primary coating layer covering the clad; and a secondary coating layer covering the primary coating layer, wherein a value of microbend loss characteristic factor F_(μBL_GΔβ) ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less when represented by F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ), wherein geometry microbend loss characteristic F_(μBL_G) (GPa⁻¹·μm^(−10.5)·10⁻²⁷) of the optical fiber is represented by $F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}},$ where κs (MPa) is a spring coefficient of the primary coating layer, H_(f) (MPa·μm⁴) is a bending rigidity of the glass portion, D₀ (MPa) is a deformation resistance of the secondary coating layer, H₀ (MPa·μm⁴) is a bending rigidity of the secondary coating layer, E_(g) (GPa) is a Young's modulus of the glass portion, E_(p) (MPa) is a Young's modulus of the primary coating layer, E_(s) (MPa) is a Young's modulus of the secondary coating layer, d_(f) (μm) is an outside diameter of the glass portion, R_(p) (μm) is a radius of an outer peripheral surface of the primary coating layer, R_(s) (μm) is a radius of an outer peripheral surface of the secondary coating layer, t_(p) (μm) is a thickness of the primary coating layer, and t_(s) (μm) is a thickness of the secondary coating layer, and wherein optical microbend loss characteristic F_(μBL_Δβ) (1/(rad/μm)⁸) of the optical fiber is represented by ${F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}},$ where Δβ (rad/m) is a propagation constant difference between propagation constant of a waveguide mode propagating through the optical fiber and propagation constant of a radiation mode, and the value of microbend loss characteristic factor is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less (excluding 5.83 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) and 5.99 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²)).
 2. The optical fiber according to claim 1, wherein the value of the microbend loss characteristic factor is 5.8 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.
 3. The optical fiber according to claim 1, wherein the value of the microbend loss characteristic factor is 3.75 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less.
 4. The optical fiber according to claim 1, wherein a coating thickness of a sum of the thickness of the primary coating layer and the thickness of the secondary coating layer is 42.0 μm or less.
 5. The optical fiber according to claim 4, wherein the outside diameter of the glass portion is 65 μm or more and 100 μm or less.
 6. The optical fiber according to claim 1, wherein the outside diameter of the glass portion is 65 μm or more and 100 μm or less.
 7. The optical fiber according to claim 1, wherein the Young's modulus of the primary coating layer is 0.08 MPa or more and 0.70 MPa or less.
 8. The optical fiber according to claim 7, wherein the Young's modulus of the secondary coating layer is 800 MPa or more and 1400 MPa or less.
 9. The optical fiber according to claim 1, wherein the Young's modulus of the secondary coating layer is 800 MPa or more and 1400 MPa or less.
 10. The optical fiber according to claim 1, wherein a propagation constant difference between LP01 mode and LP11 mode in light having a wavelength of 1550 nm is 10446 rad/m or more and 15702 rad/m or less.
 11. The optical fiber according to claim 1, wherein a MAC value (a.u.) is 5.87 or more and 7.57 or less.
 12. The optical fiber according to claim 1, wherein a mode field diameter of light having a wavelength of 1310 nm is 7.6 μm or more and 9.12 μm or less.
 13. The optical fiber according to claim 1, wherein a mode field diameter of light having a wavelength of 1310 nm is 7.6 μm or more and less than 8.6 μm.
 14. The optical fiber according to claim 1, wherein a zero dispersion wavelength is 1300 nm or more and 1324 nm or less.
 15. The optical fiber according to claim 1, wherein a zero dispersion slope is 0.073 ps/km/nm² or more and 0.092 ps/km/nm² or less.
 16. The optical fiber according to claim 1, wherein a mode field diameter of light at a wavelength of 1310 nm is 7.6 μm or more and 9.12 μm or less, a cable cutoff wavelength is 1260 nm or less, zero dispersion wavelength is 1300 nm or more and 1324 nm or less, and zero dispersion slope is 0.073 ps/km/nm² or more and 0.092 ps/km/nm².
 17. The optical fiber according to claim 16, wherein macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm is 1.5 dB/turn or less.
 18. The optical fiber according to claim 1, wherein the value of microbend loss characteristic factor is less than 5.83 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²).
 19. The optical fiber according to claim 1, wherein the bending rigidity of the glass portion is 6.48×10¹⁰ (MPa·μm⁴) or more and 8.87×10¹¹ (MPa·μm⁴) or less.
 20. The optical fiber according to claim 1, wherein macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm is 1.5 dB/turn or less.
 21. The optical fiber according to claim 20, wherein macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm is 0.2 dB/turn or less.
 22. The optical fiber according to claim 21, wherein macrobend loss at a wavelength of 1625 nm by bending at a radius of 10 mm is 0.1 dB/turn or less.
 23. An optical fiber comprising: a glass portion including a core; a clad surrounding the core; a primary coating layer covering the clad; and a secondary coating layer covering the primary coating layer, wherein a value of microbend loss characteristic factor F_(μBL_GΔβ) ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less when represented by F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ), wherein geometry microbend loss characteristic F_(μBL_G) (GPa⁻¹·μm^(−10.5)·10⁻²⁷) of the optical fiber is represented by $F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}},$ where κs (MPa) is a spring coefficient of the primary coating layer, H_(f) (MPa·μm⁴) is a bending rigidity of the glass portion, D₀ (MPa) is a deformation resistance of the secondary coating layer, H₀ (MPa·μm⁴) is a bending rigidity of the secondary coating layer, E_(g) (GPa) is a Young's modulus of the glass portion, E_(p) (MPa) is a Young's modulus of the primary coating layer, E_(s) (MPa) is a Young's modulus of the secondary coating layer, d_(f) (μm) is an outside diameter of the glass portion, R_(p) (μm) is a radius of an outer peripheral surface of the primary coating layer, R_(s) (μm) is a radius of an outer peripheral surface of the secondary coating layer, t_(p) (μm) is a thickness of the primary coating layer, and t_(s) (μm) is a thickness of the secondary coating layer, and wherein optical microbend loss characteristic F_(μBL_Δβ) (1/(rad/μm)⁸) of the optical fiber is represented by ${F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}},$ where Δβ (rad/m) is a propagation constant difference between propagation constant of a waveguide mode propagating through the optical fiber and propagation constant of a radiation mode, and the Young's modulus of the primary coating layer is 0.08 MPa or more and 0.70 MPa or less.
 24. An optical fiber comprising: a glass portion including a core; a clad surrounding the core; a primary coating layer covering the clad; and a secondary coating layer covering the primary coating layer, wherein a value of microbend loss characteristic factor F_(μBL_GΔβ) ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less when represented by F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ), wherein geometry microbend loss characteristic F_(μBL_G) (GPa⁻¹·μm^(−10.5)·10⁻²⁷) of the optical fiber is represented by $F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}},$ where KS (MPa) is a spring coefficient of the primary coating layer, H_(f) (MPa·μm⁴) is a bending rigidity of the glass portion, D₀ (MPa) is a deformation resistance of the secondary coating layer, H₀ (MPa·μm⁴) is a bending rigidity of the secondary coating layer, E_(g) (GPa) is a Young's modulus of the glass portion, E_(p) (MPa) is a Young's modulus of the primary coating layer, E_(s) (MPa) is a Young's modulus of the secondary coating layer, d_(f) (μm) is an outside diameter of the glass portion, R_(p) (μm) is a radius of an outer peripheral surface of the primary coating layer, R_(s) (μm) is a radius of an outer peripheral surface of the secondary coating layer, t_(p) (μm) is a thickness of the primary coating layer, and t_(s) (μm) is a thickness of the secondary coating layer, and wherein optical microbend loss characteristic F_(μBL_Δβ) (1/(rad/μm)⁸) of the optical fiber is represented by ${F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}},$ where Δβ (rad/m) is a propagation constant difference between propagation constant of a waveguide mode propagating through the optical fiber and propagation constant of a radiation mode, and the Young's modulus of the secondary coating layer is 800 MPa or more and 1400 MPa or less.
 25. An optical fiber comprising: a glass portion including a core; a clad surrounding the core; a primary coating layer covering the clad; and a secondary coating layer covering the primary coating layer, wherein a value of microbend loss characteristic factor F_(μBL_GΔβ) ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less when represented by F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ), wherein geometry microbend loss characteristic F_(μBL_G) (GPa⁻¹·μm^(−10.5)·10⁻²⁷) of the optical fiber is represented by $F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}},$ where κs (MPa) is a spring coefficient of the primary coating layer, H_(f) (MPa·μm⁴) is a bending rigidity of the glass portion, D₀ (MPa) is a deformation resistance of the secondary coating layer, H₀ (MPa·μm⁴) is a bending rigidity of the secondary coating layer, E_(g) (GPa) is a Young's modulus of the glass portion, E_(p) (MPa) is a Young's modulus of the primary coating layer, E_(s) (MPa) is a Young's modulus of the secondary coating layer, d_(f) (μm) is an outside diameter of the glass portion, R_(p) (μm) is a radius of an outer peripheral surface of the primary coating layer, R_(s) (μm) is a radius of an outer peripheral surface of the secondary coating layer, t_(p) (μm) is a thickness of the primary coating layer, and t_(s) (μm) is a thickness of the secondary coating layer, and wherein optical microbend loss characteristic F_(μBL_Δβ) (1/(rad/μm)⁸) of the optical fiber is represented by ${F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}},$ where Δβ (rad/m) is a propagation constant difference between propagation constant of a waveguide mode propagating through the optical fiber and propagation constant of a radiation mode, and a propagation constant difference between LP01 mode and LP11 mode in light having a wavelength of 1550 nm is 10446 rad/m or more and 15702 rad/m or less.
 26. An optical fiber comprising: a glass portion including a core; a clad surrounding the core; a primary coating layer covering the clad; and a secondary coating layer covering the primary coating layer, wherein a value of microbend loss characteristic factor F_(μBL_GΔβ) ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less when represented by F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ), wherein geometry microbend loss characteristic F_(μBL_G) (GPa⁻¹·μm^(−10.5)·10⁻²⁷) of the optical fiber is represented by $F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}},$ where κs (MPa) is a spring coefficient of the primary coating layer, H_(f) (MPa·μm⁴) is a bending rigidity of the glass portion, D₀ (MPa) is a deformation resistance of the secondary coating layer, H₀ (MPa·μm⁴) is a bending rigidity of the secondary coating layer, E_(g) (GPa) is a Young's modulus of the glass portion, E_(p) (MPa) is a Young's modulus of the primary coating layer, E_(s) (MPa) is a Young's modulus of the secondary coating layer, d_(f) (μm) is an outside diameter of the glass portion, R_(p) (μm) is a radius of an outer peripheral surface of the primary coating layer, R_(s) (μm) is a radius of an outer peripheral surface of the secondary coating layer, t_(p) (μm) is a thickness of the primary coating layer, and t_(s) (μm) is a thickness of the secondary coating layer, and wherein optical microbend loss characteristic F_(μBL_Δβ) (1/(rad/μm)⁸) of the optical fiber is represented by ${F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}},$ where Δβ (rad/m) is a propagation constant difference between propagation constant of a waveguide mode propagating through the optical fiber and propagation constant of a radiation mode, and a MAC value (a.u.) is 5.87 or more and 7.57 or less.
 27. An optical fiber comprising: a glass portion including a core; a clad surrounding the core; a primary coating layer covering the clad; and a secondary coating layer covering the primary coating layer, wherein a value of microbend loss characteristic factor F_(μBL_GΔβ) ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) is 6.1 ([GPa⁻¹·μm^(−2.5)/rad⁸]·10⁻¹²) or less when represented by F _(μBL_GΔβ) =F _(μBL_G) ×F _(μBL_Δβ), wherein geometry microbend loss characteristic F_(μBL_G) (GPa⁻¹·μm^(−10.5)·10⁻²⁷) of the optical fiber is represented by $F_{\mu{BL\_ G}} = \frac{K_{s}^{2}}{H_{f}^{2} \times D_{0}^{0.375} \times H_{0}^{0.625}}$ ${K_{s} = \frac{E_{p}d_{f}}{t_{p}}},{H_{f} = {\frac{\pi}{4}{E_{g}\left( \frac{d_{f}}{2} \right)}^{4}}},{D_{0} = {E_{p} + {\left( \frac{t_{s}}{R_{s}} \right)^{3}E_{s}}}},{H_{0} = {\frac{\pi}{4}{E_{s}\left( {R_{s}^{4} - R_{p}^{4}} \right)}}},$ where κs (MPa) is a spring coefficient of the primary coating layer, H_(f) (MPa·μm⁴) is a bending rigidity of the glass portion, D₀ (MPa) is a deformation resistance of the secondary coating layer, H₀ (MPa·μm⁴) is a bending rigidity of the secondary coating layer, E_(g) (GPa) is a Young's modulus of the glass portion, E_(p) (MPa) is a Young's modulus of the primary coating layer, E_(s) (MPa) is a Young's modulus of the secondary coating layer, d_(f) (μm) is an outside diameter of the glass portion, R_(p) (μm) is a radius of an outer peripheral surface of the primary coating layer, R_(s) (μm) is a radius of an outer peripheral surface of the secondary coating layer, t_(p) (μm) is a thickness of the primary coating layer, and t_(s) (μm) is a thickness of the secondary coating layer, and wherein optical microbend loss characteristic F_(μBL_Δβ) (1/(rad/μm)⁸) of the optical fiber is represented by ${F_{\mu{BL\_\beta}} = \frac{1}{\left( {\Delta\beta} \right)^{8}}},$ where Δβ (rad/m) is a propagation constant difference between propagation constant of a waveguide mode propagating through the optical fiber and propagation constant of a radiation mode, and the bending rigidity of the glass portion is 6.48×10¹⁰ (MPa·μm⁴) or more and 8.87×10¹¹ (MPa·μm⁴) or less. 